Abstract
Low-alloy steels containing a few percent chromium corrode less than plain carbon steels when exposed to aerated neutral waters including sea water, in spite of the general concept that the corrosion rate of carbon and low-alloy steels in natural waters is independent of their heat treatment or alloy composition. The superiority of Cr-bearing steels as tested in artificial sea water vanishes when part of the steel surface is plated with copper. This indicates that even if the rate of diffusion of oxygen toward the surface is the same, the rate of oxygen reduction thereon depends on the nature of the surface. The observed fact that the shift of the corrosion potential on stirring the water decreases with increased chromium content demonstrates that the corrosion product films formed on Cr-bearing steels provide a better barrier to oxygen diffusion. The diffusion coefficients of oxygen in the corrosion product films have been estimated using the timelag method, where time-lag was determined from the transient potential change of corroded steels upon stirring the water. D thus found is in the range of 10-6-10-7cm2/sec, the value being smaller for Cr-bearing steels.
